The diplomats, from Britain, France, Germany and the European Union, told Iran's Foreign Minister Abbas Araghchi of "their determination to use the 'snapback' mechanism -- which allows for the reimposition of all international sanctions against Iran -- in the absence of concrete progress" towards a deal on Tehran's nuclear programme "by the end of the summer", the French foreign ministry said.

They are applying pressure to convince Iran of "the urgency of returning to the diplomatic path without delay, in order to reach a robust, verifiable, and durable agreement on Iran's nuclear programme".

There has been much speculation about the status of Tehran's nuclear programme since Israeli and US strikes on Iranian military and nuclear facilities in June.

A clause in Iran's 2015 nuclear agreement, which the United States withdrew from, allows for UN sanctions against Tehran to be reimposed in case of non-compliance through a snapback mechanism.

European diplomats are seeking progress by the end of August as the agreement is due to expire in October.

Iran and the United States have held several rounds of negotiations through Omani mediators since April, before Israel launched an attack on Iran on June 13. Washington also carried out strikes on Iran, effectively ending the nuclear talks.

Since the end of the hostilities, both Iran and the United States have signalled willingness to return to the table, though Tehran has said it will not renounce its right to the peaceful use of nuclear power.

An adviser to Iran's supreme leader Ayatollah Ali Khamenei said on Monday there would be no new nuclear talks with the United States if they were conditioned on Tehran abandoning its uranium enrichment activities.